Explanation about Geostigma: A person have J-cells inside him/her. Sephiroth can use his will to make the cells "attack" the body. The body fights against it, but the effort is too much, and the person dies. That's Geostigma.
After the person dies, his/her soul is corrupted. Many corrupted souls join together, forming the Negative Lifestream.
The Negative Lifestream is nothing more than normal Lifestream corrupted by Sephiroth/Jenova's essence, and under his control. It have the same power of the normal Lifestream.
And the Lifestream is the source of all magic in FFVII, including destructive magic, like Summons, Ultima, and many others. Shinra used the Lifestream in their Mako Cannon of Junon, and it was stated in BC that a normal shot of the cannon can destroy all of Midgar. And in the end of FFVII, a portion of the Lifestream destroyed Meteor(and many buildings, as stated in "On a Way to a Smile: Case of Denzel"😉.
The Lifestream have a great power of destruction, and that's why Sephiroth was using it in AC/ACC to destroy everything in the planet.
The Lifestream is effective in any universe. It's the source of FFVII's magic afterall, and magic is effective in other universes.
And no, I never said Sephiroth can posses people. If you want to accuse me of that, you have to prove that I said it.